Analog on-channel repeaters have been used to provide coverage enhancement in FDD cellular systems. Simple analog repeaters require considerable isolation between the transmit and receive antennas, for example >120 dB. In order to achieve the required isolation, the transmitting and receiving antennas are typically isolated, either by separating them by a large distance (with connections established by cabling) or by using elaborate shielding measures.
Conventional analog repeaters are used in order to provide coverage in areas where there would otherwise be a “coverage hole” in a normal area of coverage of the cell. Isolation of the transmit and receiving antennas is particularly difficult to achieve if the coverage holes are outdoors.
State-of-the-art interference cancellation can also be employed and this provides about 30dB reduction in the amount of isolation required between the transmitting and receiving antennas. Isolation of antennas may be easier to achieve if the coverage to be provided is indoors.
All these techniques are expensive, and require careful design and planning in each and every installation.
Typically, the use of relays is limited in current cellular systems, with the primary intent of extending coverage beyond the cell boundary or for eliminating “dead spots” in the cell. A network of cells may selectively employ one or more strategically placed relays.
Normal transmission without relays is summarized in FIG. 1 for cellular communications system. Shown is a BS (base station) 10 which generates transmit signal 12 which is transmitted over a wireless channel to a UE (user equipment) 14 where it is received as signal 16. The signal level drops between the signal's transmission by the BS 10 and its reception by the UE 14. The BS 10 has a coverage area, or cell, defined by an area of acceptable signal strength.
This example assumes a single carrier operation. The forward link, the base station transmits with frequency F1 (FL) and this frequency is received at the UE 14. On the reverse link, the UE 14 transmits with frequency F1 (RL) which is then received by the BS 10.
Referring now to FIG. 2, shown as an example of a conventional on-channel analog relay scheme applied to the cellular communications example of FIG. 1. Between the BS 10 and the UE 14 is shown a relay 18. On the forward link, the relay 18 receives a signal transmitted by the BS 10 on F1 (FL) and relays this on to the UE 14 on the same forward link frequency F1 (FL). Similarly, on the reverse link, the relay 18 receives the reverse link transmission of the UE 14 on F1 (RL) and transmits this on to the BS 10 on the same reverse link frequency F1 (RL). The relay scheme of FIG. 2 suffers from the various drawbacks and complications discussed above.
The use of a single frequency translating relays is also known. One of the two frequencies used between the BS and the relay or between the relay and the UE is obtained from outside the cellular spectrum (e.g., microwave frequency between BS and relay and cellular frequency between relay and UE). This implies additional licensing cost to the operator. Even if cellular frequencies are used between the BS and relay and between the relay and UE, a UE has to perform handoff in migrating from the frequency it was using to communicate with the BS, to that of the relay.